The invention proceeds from a sodium high-pressure lamp in accordance with the preamble of claim 1. In particular, these are sodium high-pressure lamps having a starting aid fitted outside on the ceramic discharge vessel.
A sodium high-pressure lamp having an external starting aid is already known from EP-A 592 040. When sodium high-pressure lamps are started, the electric flashover is performed by a high-voltage pulse which is applied between the two electrodes located in the ceramic discharge vessel. The absolute value of this high voltage is determined by the geometrical dimensions of the discharge vessel and, in particular, by the cold filling pressure of the inert gas (mostly xenon) located therein. A high cold filling pressure leads, on the one hand, to high light yields and good maintenance, but it requires, on the other hand, correspondingly high starting voltages, which are not readily available.
An electrically conducting, metallic starting aid fitted outside on the discharge vessel provides a remedy. It is either a separate part or a strip which is sintered onto the ceramic discharge vessel. The separate part can take the form of a wire bearing against the discharge vessel, or of a helix made from high-melting metal. It is pressed on by means of a bimetal. The bimetal lifts this starting aid off the discharge vessel during operation. This is required, since the starting aid is electrically connected to one of the two electrodes, and there is therefore present between the starting aid and the second electrode a high electric field strength which leads to the diffusion of the sodium through the wall of the discharge vessel. However, the bimetal has the disadvantage that it is difficult to mount. Moreover, after some time it can tire or become detached, and thus lead to premature lamp failures.
A bimetal is dispensed with in the case of starting aids without direct electrical contact with the line voltage. Instead of this, use is made of axial starting strips and a closed ring around each electrode. The starting aid is coupled in this case only capacitively to the starting pulse, this coupling depending on the area of the starting aid and the spacing between the starting aid and electrode. The diffusion of sodium is prevented since said aid is at a freely floating potential.
In the case of a capacitive starting aid without contact, the potential of the starting aid depends on the voltage divider which is formed by the capacitances between the starting aid and the electrodes. A role is also played by high-resistance, electrical connections between the starting aid and the electrodes owing to the finite conductivity of the ceramic and along the ceramic. In the case of a symmetrical design, the potential of the starting aid will be in the middle between the frame and the potential of the starting pulse. It then follows that only half the voltage value is available for the flashover between the electrode and wall of the discharge vessel. The flashover can occur both between the frame electrode and the wall with the starting aid, and between the electrode at high voltage and the wall with the starting aid. The point is that there is a similar potential difference in both cases.
The flashover forms in like manner in both cases, that is to say in the case both of direct and of capacitive coupling. Initially, a discharge is produced between the first electrode, at which the high-voltage pulse is present, and the adjacent site of the ceramic wall, on which the starting aid is seated on the outside. The discharge propagates along the starting aid on the ceramic wall, until the flashover for the second electrode finally occurs.
Without direct electrical contact with the line voltage, because of the capacitive coupling a potential is set up at the starting aid which is between that of the high-voltage pulse at the first electrode and the zero potential of the second electrode. The potential difference between the high-voltage pulse and the starting aid is therefore less than when the starting aid is at the potential of one of the electrodes. That is to say, the starting voltage requirement is substantially increased in the case of a starting aid without contact.
A particular disadvantage of the sintered-on, capacitive starting aid known from EP-A 592 040 and having two rings resides in that applying the rings is a complicated procedure in production engineering, and this occasions high costs. Moreover, the required absolute value of the starting voltage is relatively high. On the other hand, a simple, cost effective starting strip does not start reliably enough, and the required absolute value of the starting voltage is still substantially higher than in the case of the ring solution.
It is the object of the present invention to provide a sodium high-pressure lamp in accordance with the preamble of claim 1 which starts without any problem and whose starting aid can be produced simply and cost effectively.
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
The invention dispenses with a closed ring (complete circle) around the electrode at the end of the starting strip.
Instead of this, an elongated starting strip is widened at the level of each electrode by a coupling surface so that the capacitance which is formed by the electrode with the coupling surface is substantially increased. As a result, there is a substantial rise in the charge quantity which flows between the electrode and the inner surface of the ceramic tube upon application of the starting pulse, and a stronger ionization of a gas is produced. The coupling surface has a maximum transverse extent corresponding to a center angle xcex1 of at most 180xc2x0, in particular 50xc2x0 to 120xc2x0.
By contrast with the starting strip with two conductive rings, this design has the advantage of producing an asymmetric field strength distribution on the electrode. For this reason, the voltage required for a flashover is substantially less than in the case of the symmetrical ring arrangement. The second design with the transverse strips is intended to avoid the loss in field strength which can occur owing to the skewed position of the electrodes, which cannot be completely ruled out in the production process: the minimum electrode-transverse strip spacing responsible for the field strength produced on the electrode increases only insubstantially even when the electrode is situated off the axis, with the result that the scatter thereby caused in the starting voltage is reduced. An advantageous configuration of the starting aid consists in that the coupling surfaces are constructed at both ends of the starting strip as conductive transverse strips which extend over less than half the circumference of the discharge tube.
The electrode advantageously comprises a shaft of given diameter D and a wider part, in particular a helix or a sphere, of greater transverse dimension, the coupling surface being fitted in the region of the wider part.
The coupling surface according to the invention increases the capacitance with respect to the electrode and produces a particularly powerful starting spark. Moreover, it is much easier to apply than a complete circle. Detailed investigations have shown that given an area of the same magnitude the starting voltage required for a complete circle is greater than in the case of a partial circle, in particular with a center angle xcex1 of less than 180xc2x0. It is to be surmised that the missing symmetry of this arrangement of the starting aid and electrode gives rise to an inhomogeneous field distribution with particularly high peak field strengths on the electrode, which exceed the peak field strengths of the symmetrical arrangement (with rings) by a few percent (up to 5%). The larger peak field strengths facilitate a flashover, and therefore the required starting voltage is reduced. The smaller the center angle selected, the greater the asymmetry, and therefore the peak field strength. In the case of very small center angles below 45xc2x0, however, the capacitive coupling declines once again. A center angle of 50 to 120xc2x0is therefore optimum. The coupling surface can preferably have a square, elliptic or circular surface and, in particular, be a transverse strip in the shape of a partial circle.
It holds in essence that: the larger the capacitive area in the vicinity of a first electrode, the more easily the flashover can occur between this electrode and the wall of the discharge vessel.
The starting aid presented here can be produced simply and cost effectively, for example by means of a screen printing or stamp printing method, without the need for complicated manipulation or rotation of the discharge vessel. In addition, the starting voltage is lower in conjunction with the same coupling surface at the strip ends by comparison with the prior art. The reason for this is the inhomogeneity of the electric field strength between the coupling surface and electrode. By contrast, the known ring-shaped arrangement produces a lower electric field overall at the electrode because of its radial symmetry. Consequently, the starting voltage applied can be lower in the case of the present invention, in order to produce the field strength required for the flashover at the electrode.